Code formatter

Code Formatter

Configures and applies language-specific formatters and linters to establish consistent code style across a codebase — generating configuration files, fixing violations, and setting up pre-commit hooks and CI checks to enforce formatting automatically.

When to Use

• User asks to "format this code", "fix linting errors", or "set up Prettier"
• A new project needs formatter/linter configuration
• CI is failing due to formatting violations
• The team wants to enforce consistent code style in PRs
• Existing linting config needs to be updated or extended
• User asks to add pre-commit hooks for automatic formatting

Process

1. Detect the language(s) and existing tooling:

   • JavaScript/TypeScript: Prettier, ESLint (with @typescript-eslint, eslint-plugin-react, etc.)
   • Python: Black (formatter), Ruff (linter + formatter), isort, mypy, flake8
   • Go: gofmt (built-in), goimports, golangci-lint
   • Rust: rustfmt, clippy
   • Java: google-java-format, Checkstyle, Spotless
   • Ruby: RuboCop, StandardRB
   • Check for existing config files: .eslintrc.*, prettier.config.*, pyproject.toml, .rubocop.yml

2. Generate formatter configuration:

   Prettier (.prettierrc or prettier.config.js):

   • printWidth: 100 (adjust to team preference)
   • tabWidth: 2 for JS/TS, 4 for Python conventions
   • singleQuote: true/false based on existing codebase
   • trailingComma: "all" (ES5+ friendly)
   • semi: true/false
   • endOfLine: "lf"
   • Add overrides for specific file types (JSON, YAML, markdown)

   ESLint (eslint.config.js or .eslintrc.json):

   • Extend from eslint:recommended, @typescript-eslint/recommended
   • Add framework-specific plugins (React, Vue, import ordering)
   • Configure rules based on existing codebase conventions
   • Add no-console (warn in dev, error in prod)
   • Set no-unused-vars to error
   • Add import/order for consistent import ordering

   Black + Ruff (pyproject.toml):

   • line-length = 100
   • target-version = ["py311"]
   • Ruff rules: E, F, I (isort), N (naming), UP (pyupgrade), B (bugbear)
   • Exclude: migrations/, __pycache__/, .venv/

3. Generate .prettierignore / .eslintignore:

   • Exclude: dist/, build/, node_modules/, coverage/, auto-generated files, vendored code

4. Add formatter scripts to package.json or Makefile:

   • format: run formatter to fix all files
   • format:check: check without modifying (for CI)
   • lint: run linter, exit non-zero on violations
   • lint:fix: run linter with auto-fix

5. Set up pre-commit hooks (.pre-commit-config.yaml or lint-staged + Husky):

   • Run formatter and linter only on staged files (fast)
   • Block commit if violations remain after auto-fix

6. Set up CI check:

   • Add a lint job to the CI pipeline that runs format check and linting
   • This ensures formatting is enforced on all PRs

7. Fix existing violations if asked:

   • Run the formatter on the provided code and show the corrected output
   • Explain non-trivial lint rule violations (not just formatting)

Output Format

.prettierrc

{
  "printWidth": 100,
  "tabWidth": 2,
  "singleQuote": true,
  "trailingComma": "all",
  "semi": true,
  "endOfLine": "lf",
  "arrowParens": "always",
  "overrides": [
    { "files": "*.json", "options": { "printWidth": 80 } },
    { "files": "*.md", "options": { "proseWrap": "always" } }
  ]
}

eslint.config.js (ESLint v9 flat config)

import js from '@eslint/js';
import tseslint from 'typescript-eslint';

export default tseslint.config(
  js.configs.recommended,
  ...tseslint.configs.recommended,
  {
    rules: {
      'no-console': ['warn', { allow: ['warn', 'error'] }],
      'no-unused-vars': 'off',
      '@typescript-eslint/no-unused-vars': ['error', { argsIgnorePattern: '^_' }],
      '@typescript-eslint/explicit-function-return-type': 'off',
      'prefer-const': 'error',
    },
  },
  { ignores: ['dist/', 'build/', 'node_modules/'] }
);

pyproject.toml (Ruff + Black)

[tool.black]
line-length = 100
target-version = ["py311"]

[tool.ruff]
line-length = 100
target-version = "py311"
select = ["E", "F", "I", "N", "UP", "B", "C4"]
ignore = ["E501"]  # line length handled by Black
exclude = ["migrations/", ".venv/"]

Examples

Example Input

const x=1
const foo = function(a,b,c){
return a+b+c}

var unused = "hello"

Example Output (Prettier + ESLint applied)

// After Prettier formatting:
const x = 1;
const foo = function (a, b, c) {
  return a + b + c;
};

// ESLint violations (after formatting):
// Line 5: 'unused' is assigned a value but never used. (@typescript-eslint/no-unused-vars)
// Line 1: Prefer 'const' over 'let'. (prefer-const) ✓ already const
// Line 2: Prefer arrow function expression. (prefer-arrow-callback) — optional rule

Boundaries

• Do NOT apply formatting to auto-generated files (migrations, protobuf output, OpenAPI generated clients).
• Do NOT change linting rules that would cause existing passing CI to fail without flagging the breaking change.
• Do NOT apply eslint --fix to logic-changing rules (like eqeqeq changing == to ===) without reviewing each case.
• When configuring ESLint, do NOT disable security-relevant rules (no-eval, no-implied-eval, etc.).
• Do NOT generate formatter configs that conflict with each other (e.g., Prettier and ESLint formatting rules overlapping — use eslint-config-prettier to disable conflicting ESLint formatting rules).
• If multiple languages are used in the repo, generate separate formatter configs per language — do not try to handle all with one tool.